1. Field of the Invention
The present invention relates to an optical pickup for recording information on a recording layer of an optical-type information recording medium such as optical disc and for retrieving (playing back) information from the recording layer by radiating a convergent light beam on the recording layer, and more particularly to an optical system used for such optical pickup.
2. Description of the Related Art
In recent times, optical discs are widely utilized as means for recording and retrieving information or data (e.g., image/video data, voice/sound data, and computer data). For example, high density recording type optical discs which are called DVDs (digital versatile discs) are marketed.
In order to increase the recording capacity, a next generation optical disc system is studied. In such optical disc system, it is desired that a violet laser having a short wavelength is used as a light source, and an optical pickup includes an object lens having a large numerical aperture (e.g., 0.8 or more). This would make it possible to record data (or information) on an optical disc at a density greater than DVD.
In general, a cover layer is placed on an uppermost recording layer of an optical disc. The cover layer is a light transmissive layer. When an object lens has a large numerical aperture, spherical aberration often occurs if a thickness of the cover layer deviates from a reference (standard) value, because the light beam passes through the cover layer when recording and retrieving the data. If the spherical aberration occurs in a great quantity, the object lens cannot show an originally designed (expected) spatial frequency characteristic (MTF: modulation transfer function). In particular, jitter increases due to irregularity in the cover layer thickness when short marks and pits are recorded and scanned (read, retrieved).
In order to solve such inconvenience, it is necessary to correct (compensate for) the spherical aberration resulting from the cover layer thickness irregularity. It is also necessary to correct another spherical aberration resulting from the object lens shape irregularity, which inevitably occurs during an object lens manufacturing process. To this end, use of an expander lens assembly, which has two lens (concave lens and convex lens), in an optical pickup is proposed in for example Japanese Patent Application Kokai No. 2000-131603. Alternative resolution can be found in Japanese Patent Application Kokai No. 11-259906, which has a mechanism to move a collimator lens.
When the expander lens assembly is used to correct the spherical aberration, a parallel light beam is transformed to a convergent light beam, a divergent light beam or another parallel light beam having a slightly different diameter by the expander lens assembly before the light beam is incident to the object lens, so that the light beam which has passed through the object lens has certain spherical aberration to counterbalance another spherical aberration which will be generated at the cover layer. If the cover layer has the reference thickness, the expander lens assembly changes (transforms) the parallel light beam to another parallel light beam having a slightly different diameter before the light beam enters the object lens. If the object lens is designed such that spherical aberration produced when the light beam passes through the object lens will be counterbalanced by spherical aberration produced at the cover layer having the reference thickness, then a spot created on a recording layer by the convergent light beam which has passed through the cover layer has no spherical aberration. On the other hand, if the cover layer's thickness deviates from the reference value, the expander lens assembly changes the parallel light beam to the convergent or divergent light beam before the light beam enters the object lens. The expander lens assembly changes the light beam such that spherical aberration produced when the convergent or divergent light beam passes through the object lens will be counterbalanced by spherical aberration produced at the cover layer. As a result, there is no spherical aberration on the recording layer.
For some of the high-density optical disc systems, it is desired to use a DPP (differential push pull) method for detection of tracking errors, which utilizes multiple beams, or to use a CTC (cross talk canceler) method for high density recordation.
If the optical pickup includes an optical system for spherical aberration correction such as expander lens assembly, and multiple beams are used, then spot-to-spot intervals (spot distance) of the multiple beams on the recording layer change upon actuation of the spherical aberration correcting optical system.
The change in the beam spot intervals deteriorates sensitivity of the tracking error detection in the DPP method, and changes a delay time between a main beam and a sub-beam in the CTC method.